1. Field of the Invention
The present invention relates to a subscriber connection circuit for disconnecting, at the exchange end, a subscriber connection line having feed resistors which can be bridged in low-ohmic fashion and by way of which the line conductors of the subscriber connection line are connected, either in high-ohmic fashion or low-ohmic fashion, depending upon the operating state, to one of the two poles of the feed voltage source. The circuit also comprises an indication circuit which effects a plurality of different indication function as a result of the evaluation of the potential state prevailing at line-wire related fixed coupling points, and wherein, accordingly, different response switching thresholds are implemented, which moreover is connected at its output to a filter element in the event of the execution of specific indication functions, and which is preferably used for the control of a plurality of subscriber connection circuits.
2. Description of the Prior Art
In a subscriber connection circuit of the type mentioned above (German published application 29 39 009, fully incorporated herein by this reference) it is possible to ensure that the feed current source of the telephone exchange which services a plurality of subscriber connection circuits is not subject to overload, since the maximum feed current is not received in all operating states. In the subscriber connection circuit, in the rest state highly-ohmic resistors are contained in the feed circuit in which case the feed current which then flows is sufficient to facilitate an excitment indication. In the conversation state and in the rest phase these highly-ohmic feed resistors are bridged, at least when the resistor is located in the one line wire of the subscriber connection line, enabling the flow of a full feed current, or at least a higher feed current, then in the rest state.
In this subscriber connection line it is also possible to prevent an overlong actuation of the ground key in the subscriber apparatus, leading to overloading of the switching elements contained in the feed circuit, as in such a case switch-over must again be effected from the low-ohmic feed to the highly-ohmic feed.
In the aforementioned known circuit arrangement, the individual indication functions, therefore, for example, the excitation indication, the indication of a loop closure due to a call and, in the case of dailing, can be carried out regardless of the different feed conditions which prevail during different operating states and regardless of the different requirements on the analysis feed, using a single indication circuit, in the corresponding different response switching thresholds are, in each case, implemented in this indication circuit. A filter element which is connected at the output, in the event of specific indication functions of the indication circuit, serves to increase analysis reliability.
In particular in the case of the indication of the loop closure as a result of a call, it is important that the indication result should be availiable as quickly as possible following the instant of the loop closure since, for example, when the subscriber loop is closed the carbon microphone of the subscriber apparatus is traversed by the feed current which is superimposed upon the ringing alternating current, so that the danger of overload exists and, accordingly, the call must be disconnected as directly as possible following the loop closure.
This means that it is advisable to recognize, as soon as possible, even those loop closures which occur during a ringing interval in order to be able to introduce the necessary switching measures.
In the aforementioned circuit arrangement, the ringing phase and the ringing interval are treated as one and the same operating mode as regards the switching technology, which differ control-wise merely in that in the ringing phase the ringing relay by way of which the ringing voltage is connected has pulled up, whereas in the ringing interval it has dropped. Therefore, there is no switch-over of the switching element by way of which the threshold voltages are connected to the indication circuit during the various operating states. Since in the operating state ringing phase/ringing interval the feed voltage is supplied via a voltage divider which, during the ringing phase, is connected to the feed voltage superimposed by the ringing alternating voltage, but in the ringing interval is connected solely to the feed voltage, and appropriate threshold value occurs during the ringing interval and prevents the response of the indication circuit. It has already been proposed that the same conditions be set up during the ringing interval as in the "conversation/dial reception" operating state during which the filter element is not actuated. However, this can lead to the erroneous response of the indication circuit in the event of a change from the ringing phase to the ringing interval because of interference pulses and compensation processes on the line.